Using an observatory on the edge of the Tibetan Plateau, astronomers have spotted the highest energy light ever, gamma ray photons up to 1.4 petaelectronvolts . They have traced these extreme photons back to a dozen of their likely sources: powerful factories in the Milky Way Galaxy that accelerate charged particles called cosmic rays. The results are challenging theorists' understanding of what these factories are and how they generate such high-energy light.
Discovered more than 100 years ago, cosmic rays are charged particles, including protons and other atomic nuclei, that have been accelerated nearly to the speed of light. Their sources are poorly understood because interstellar magnetic fields bend them on their path to Earth. However, as cosmic rays rocket away from their sources, they also emit photons, usually about one-tenth as energetic as the cosmic rays themselves, that follow a straight path to Earth.
These observations will leave theorists scratching their heads to explain how PeVatrons actually work. For instance, models have predicted strong magnetic fields from the pulsar in the Crab nebula can boost particles to 0.1 PeV, but to reach 1 PeV, Cao says, all the parameters need to be pushed to the extreme.